Process for the heat treatment of noncementable metals



Patented June 20, 1933 UNITED STATES PATEN OFFICE WALTER BECK, OI IRANKIORT-ON-THE-MAIN, GERMANY, ASSIGNOB, BY MESNE AS- SIGNMENTS, TO B. I. DU POINT DE NEMOUBS AND COMPANY, A CORPORATION OF DELAWARE Y PROCESS FOR THE BEAT TREATMENT OF NONOEMENTAIBLE METALS No Drawing. Application filed December 17,

This is a continuation in part of my copending application S. N. 279,197, filed May 19, 1928 Patent No. 1,799,945 issued April 7, 1931. In S. N. 279,197 I have described a method for the thermal treatment or improvement of metals and alloys by means of heating the same in fused salt baths containin oxidizable substances, such as finely divlded carbon, metal cyanide, or substances which are capable of forming these materials under the conditions of operation, as e. g., carbonizable organic substances. These oxidizable substances are capable of reducing to metal the oxides or other compounds of the metal which may form during the heat treatment. 7

I have now found that the process of thermally treating tioned process is especially satisfactory when applied to metals or alloys which do not undergo cementation, such as that which occurs with iron or steel. Such metals or alloys are, for example, brasses, German silver, bronze, copper, silver and the like. By the term, noncementable metals, as used in this specification and its appended claims, I wish to include both metals and alloys which do not undergo cementation when heated in the presence of carbon or carbonaceous materials. 7

While it is known that fused salt baths for thecementation of iron and iron alloys generally contain 40-50% of alkali metal cyanide, I have found that quantities of cyanide salt or alkalimetal cyanide, such as sodium cyanide, far below 10% and between 5 and 1% or even lower than this can be used for the successful thermal improvement of noncementable metals. In most cases, quantities of about 1% have been added and have proved satisfactory.

Preferably I add some finely divided charcoal or materials which will produce carbon during the heat treatment such as saw dust, cellulose scraps or the like, to the fused salt baths or the salt mixtures used for the baths, in order to prevent premature oxidation of the oxidizable constitucuts of the bath by the action of the air.

Inasmuch as superficial oxidation takes very thin layer of salt,

metals by my above men- 1928, Serial No. 326,707, and in Germany December 20, 1927.

place when the objects are taken out of the ighly heated bath, and dipped intoa cooling bath, it is a surprising fact that the which clings to the object .upon separation from my molten bath, is capable of protecting the object completely from oxidation. But this is the case, in spite of the presence of the small quantities of oxidizable substances which can use in my process.

The 'noncementable metals can be thermally treated by my process to impart to them the desired properties such as hardness, toughness, elasticity, softness etc. in a degree which dependsupon the specific conditions under which the process is carried out, such as temperature duration of heating, subsequent treatment, such as cool-- ing, quenching or the like.

The metals or alloys or the objects prepared therefrom, such as sheets, plates, and

the like, which are treated in accordance with my invention, show a very bright appearance after cooling down; and indeed, even when they-have been introduced into the fused bath in an already superficially oxidized condition. The present process is suitable, therefore, not only for the improvement of the internal structure of noncementable metals, but also for the bright annealing of metals and alloys to give them a bright surface appearance.

The articles thus treated may be worked on by rolling, pressing or the like without requiring any intermediary scouring or pickling processes in order to remove scale. This scale is usually formed on the surface during the heat treatment when the latter is carried out in the ordinary way.

Thus my new process not only saves labour as well as chemicals but also avoids loss of material by doing away with the need for a pickling process wherein some metal or alloy is necessarily dissolved and thereby removed.

According to one way of carrying out my invention, the metals or alloys to be improved areput through a thermal pre-treatment using other methods, particularly by heating in gases containing little or no naces, t

further working process. A

0 gen and then improving b my method oi using a salt bath. One min, 0. g first heat the objects to be improved with hot ofigases such as those from smelt furhus pre-heating to the desir temperature, and then introduce the objects while at this temperature into the molten bath. By this method of working, perficial oxidation produced b the retreatment is overcome in the sed bath. An advantage of preheatin is that lowering of the temperature of t e molten bath, due to bringing the cold objects into it, does not take place, thus increasing the efii, ciency of the bath.

Example 1.A German silver sheet is dipped into a bath containing 48 parts potassium carbonate, 48 parts sodium chloride, 2 parts sodium cyanide and 2 arts finely pulverized wood charcoal. The ath is kept at about 800 C. The immersed sheet is left in the bath for about 15 to 20 seconds, then lifted out and ueuched in water. The sheet, the surface 0 which is quite bright, may be worked on by rolling, pressing, stamping or the like.

Example 2.--A brass ingot is immersed in a bath of 47 parts sodium carbonate, 47 parts potassium chloride, 3 parts sodium cyanide and 3 parts animal charcoal at a temperature of about 630 C. The ingot after having taken on the temperature of 630 C. is quenched in the ordinary way in water and may then be sub'ected to any er the heat treatment the ingot is bright and soft.

What I claim is:

1. Process for heat treating non-cementable alloys containin zinc, comprising heating said alloy in a bath com rising an alkali metal carbonate, an alka imetal chloride, 1 to 5% alkali metal cyanide and charcoal at about 630 to 800 C.

2. A salt bath for heat treating non-cementable alloys containin zinc comprising a major portion of su tantially equal weights of alkali metal chloride and carbonate, about 1 to 5% alkali metal cyanide and about 1 to 3% finely divided charcoal.

Signed at Frankfort-on-the-Main, Germany, this 28tl1 day of November, A. D. 1928.

WALTER BECK.

any su- 

